At present, the main current of polycarbonate production is interfacial polycondensation that starts from phosgene and bisphenol A and uses a reaction solvent of methylene chloride.
However, phosgene to be used in the interfacial polycondensation is highly toxic, and methylene chloride to be used therein is a halide solvent of which the use shall be much restricted for the protection of the environment. Therefore, the development of novel techniques substitutable for this is desired.
Recently proposed were melt interesterification (transesterification) and solid-phase polymerization.
The melt interesterification is to polymerize a dihydroxy compound and a dicarbonate compound through interesterification of the two in the presence of no solvent; while the solid-phase polymerization is to polymerize a prepolymer under heat in a solid phase.
It is expected that those new techniques solve the problems of the conventional interfacial polymerization and stably give polycarbonate of high quality.
Some methods of solid-phase polymerization have been disclosed, in which a starting, amorphous polycarbonate prepolymer is processed with a solvent or heated for crystallization to make it have a degree of crystallinity of about 20% (melting point: 190 to 210.degree. C.), and then polymerized in a solid phase at about 200.degree. C. or so in an inert gas stream of, for example, nitrogen, argon, helium or carbon dioxide (see Japanese Patent Application Laid-Open (JP-A) Nos. Sho-63-223035, Sho-64-16826, Sho-64-38433). In those, the starting prepolymer is crystallized prior to the solid-phase polymerization to have an elevated melting point so that it can be polymerized in a solid phase at high temperatures within a shortened period of time.
However, where the crystallization treatment is effected under heat in an inert gas stream such as that mentioned above, it is said that it takes a long period of time of from several tens hours to several hundreds hours since the crystallization rate of the prepolymer is very low (see Polycarbonate Resin Handbook, published by Nikkan Kogyo Shinbun Co.).
As in the above, the crystallization treatment is an indispensable pre-treatment for the conventional solid-phase polymerization.
The conventional solid-phase polymerization is effected in an inert gas stream (see Japanese Patent Publication (JP-B) No. Hei-6-99551). The inert gas includes nitrogen, carbon dioxide, argon and helium. However, the essential function of the inert gas used is to lower the partial pressure of the side product formed in a gaseous phase, or that is, to control the diffusion of the side product into the particulate prepolymer being polymerized.
Having known the prior art as above, we, the inventors herein provide the present invention, of which the object is to provide improved processes for producing polycarbonate through solid-phase polymerization. The processes of the invention simplify the step of pre-treatment for crystallization of prepolymers to be effected prior to the solid-phase polymerization, and the time for the processes is much shortened.